The present invention is related to the field of manufacturing processes for force sensors which include a transducer (sensing element). More specifically, the present invention is related to the field of manufacturing processes for force sensors in which a relatively flexible diaphragm is bonded to a base to form a transducer and deflections of the diaphragm with respect to the base produced in response to sensed force result in varying electrical properties of the transducer which are representative of the sensed force. Preferably the sensed force corresponds to pressure.
Pressure transducers are known in which a relatively flexible diaphragm is bonded to a base and deflections of the diaphragm with respect to the base in response to sensed pressure results in altering electrical properties of the transducer. Such transducers can comprise either capacitive pressure transducers or resistive pressure transducers wherein in both cases deflection of the diaphragm with respect to the base results in altering either a capacitance or resistance output, respectively, of the transducer in response to sensed pressure. Such capacitive pressure transducers generally comprise either ceramic capacitive transducers, in which a thin ceramic plate having an electrode thereon is displaceable with respect to an electrode mounted on a base ceramic substrate, or silicon capacitive pressure transducers in which a silicon diaphragm, acting as one capacitor electrode, is selectively displaceable with respect to an electrode provided on a mating glass base. Copending U.S. patent application Ser. No. 696,933 referred to above and issued U.S. Pat. No. 4,384,899 to Myers both illustrate silicon capacitive pressure transducers corresponding to the above-noted structure. In the Myers patent, metallized vias in a glass base of the silicon capacitive transducer are utilized as electrical output connections for the capacitor electrodes. In the above-noted copending U.S. patent application, embedded wires in the base are utilized for the electrical output connections of the electrodes. In both cases, the silicon diaphragm is relatively thin with respect to the glass base, and the diaphragm and base are anodically bonded to form a transducer (sensing element) having a capacitance which varies as a function of sensed pressure.
In silicon capacitive pressure transducers, such as those discussed above, it has been found that to obtain a desired sensitivity of the transducer with respect to sensed pressure, the silicon diaphragm must be extremely thin. Prior silicon diaphragms are, therefore, either etched or ground to a predetermined relatively-thin thickness dimension prior to their being bonded to an associated glass base. However, this prior manufacturing technique required careful handling of the thin silicon diaphragm to prevent rupturing of the diaphragm during the manufacturing process. In addition, the sensitivity of the resultant transducer often did not correspond to theoretical predictions. This was due to manufacturing tolerances with respect to the thickness of the silicon diaphragm and might also be due to the fact that the anodic bonding of the silicon diaphragm to the glass base might be variable and thereby affect the transducer sensitivity. Therefore, the prior manufacturing process was not substantially efficient since pressure transducers having the desired sensitivity (capacitance or resistance variation as a function of pressure) could not be readily produced unless extreme manufacturing care was exercised and very close tolerances were instituted with regard to providing an exact thin silicon diaphragm and then bonding this diaphragm to a glass substrate.